Optical lithography, i.e., photolithography, has enabled the exponential growth of the semiconductor industry in recent years. The increased resolution of features on integrated circuits has partly resulted from the increase in the numerical apertures of the lenses of optical lithography systems and the decrease of the exposure light wavelengths of the systems. Today, many optical lithography advances, such as phase-shift photo masks, multi-step patterning, new laser and x-ray sources, and Langmuir-Blodgett films, have allowed for further improvements in feature resolution.
Projection exposure systems developed during the late 1980's were a turning point for integrated circuit fabrication. In such systems, a pattern defined by a mask was projected by an optical system onto a wafer and the mask was typically the same size as the desired pattern. Due to limitations in the complexity of these systems, only part of the pattern was exposed on the wafer at any one time and, therefore, such systems were limited to resolutions between 0.7 μm and 3 μm.
As a result, in the early 1990s, step-and-repeat exposure systems were introduced. The masks of these systems only contained the pattern of a few dies instead of a complete wafer and allowed for the masks to be magnified by a factor of four relative to the pattern formed on the wafer. As such, these systems were used for lithography resolutions between 0.25 μm and 0.7 μm.
In the late 1990s, another evolution in optical lithography was the transition to step-and-scan exposure systems. These systems moved the reticle stage and wafer stage in directions normal to one another during exposure and projected an image onto a wafer through a single slit. These systems enabled higher resolutions with better control. Step-and-scan systems are currently being used for the manufacturing of integrated circuits in the 32 nm to 45 nm technology node.
For scaling beyond the 32 nm technology node, multiple-patterning lithography has been developed. Multiple-patterning lithography can be implemented with different manufacturing processes, such as litho-etch-litho-etch requiring two etching steps, litho-litho-etch, and spacer double-patterning. As such, multiple-patterning lithography requires two or more masks individually developed on the wafer and, therefore, mask production for these systems adds considerable expense to the production of integrated circuits.
Therefore, a new, more cost effective system for increasing lithography resolution is needed.